The present invention relates to coherent optical range finders and, in particular, it concerns a coherent range measurement system where a single detector is used to detect time-multiplexed signals reflected from a plurality of directions.
Coherent laser range detection is based on interference of two light beams on a detector. The first light beam originates directly from a laser (defined as the “local oscillator”) while the second light beam is the reflection from a laser illuminated target (defined as the “signal”). This method is very sensitive and even weak reflections from the target can be detected. This method is also characterized by being very insensitive to background radiation.
Unfortunately, this method requires a very complicated and expensive optical setup. Utilization of optical fibers somewhat reduces the complexity and improves robustness.
An optimal receiver that uses polarization splitters and balanced receivers can achieve shot noise limited performance. However, this receiver is complicated and expensive.
Present coherent range detection systems are expensive; therefore, direct detection methods are used whenever possible.
An example of prior art coherent receiver is presented in FIG. 11A continuous laser source 5 is modulated to pulses by a modulator 10, and the pulses are then amplified 15 and pass through the circulator 20 to be transmitted by optics 25. The received signal from the target is collected by optics 25 and directed by circulator 20 to a different path 30 onto the coherent receiver 35. Some of the light from the continuous laser 5 is coupled 40 directly onto the coherent receiver in order to generate interference. The receiver 35 can include polarization splitters and balanced receivers in order to achieve optimal detection.
This system is complicated and expensive. The use of separate modulator (not modulated laser) and interfering CW light with pulsed signal dictates expensive components.
An alternative approach has been suggested in U.S. Pat. No. 6,469,778 to Asaka et al. which is hereby incorporated by reference in its entirety. Asaka et al. proposes splitting the transmitted illumination pulse and using a part of the radiation as a local oscillator for coherent detection.
There is a need for a coherent detection system which is simple, inexpensive and can be scalable to multiple transmitters and receivers even at the price of non-optimal operation.